1. Field of the Invention
This invention relates to a magnetic disk apparatus for recording the information, and particularly relates to a magnetic disk apparatus that uses a perpendicular magnetic recording medium that holds the data by means of the magnetic information of the direction perpendicular to the layer surface.
2. Description of Related Art
In the conventional disk apparatus, a magnetic head is moved in the radial direction with respect to a rotating magnetic disk so that the magnetic head is positioned accurately at a target data track, and the information is written and read magnetically. A magnetic disk has a servo area for positioning a magnetic head to a track. One example of a servo pattern recorded on a servo area is shown in FIG. 18. A read element of a magnetic head reads a servo pattern from left to right relatively in FIG. 18 with rotation of a disk. The servo pattern has a track code area on which the track number information for obtaining approximate head position signal on the entire disk surface and a burst area for obtaining accurate position information on each inside track. The burst area comprises a pair of A-burst and B-burst and a pair of C-burst and D-burst repeating alternately with the track width. The pair of A-burst and B-burst is disposed with deviation of a half of a track width from the pair of C-burst and D-burst. This structure is similar to the technique disclosed in JP-A No. 222468/1983, and this technique is applied to magnetic disk apparatus popularly. Furthermore, the servo pattern has a continuous pattern called as preamble area to reduce the effect of magnetic characteristic and floating of a disk. Furthermore, the servo pattern has an address mark area to detect timing of end of the preamble area and timing of start of the track code area and following pattern.
FIG. 19A is a read waveform obtained at the position of the read element shown in FIG. 18. For simplification, read waveform of the address mark area and track code area is omitted. Burst amplitudes of A-burst to D-burst are normalized by use of amplitude of preamble, and the amplitude difference between A-burst and B-burst shows N-position signal. Similarly the amplitude difference between C-burst and D-burst shows Q-position signal. FIG. 19B shows the change of N-position signal and Q-position signal. As shown in FIG. 19B, N-position signal is equal to 0 at the position where the read element extends over A-burst and B-burst equally, and N-position signal varies approximately in proportion to the deviation from the position. The servo circuit controls a rotary actuator 13 with targeting the position where N-position signal is adjusted to 0 so that the center of a track is traced.
Patent Literature 1: JP-A No. 222468/1983
Patent Literature 2: JP-A No. 230734/2002
Patent Literature 3: JP-A No. 150729/2002
It is required to develop high memory capacity magnetic disk apparatus that a small bit is formed on a magnetic disk with smaller magnetization pattern. Furthermore, it is required that the formed small bit remains stable for a long time. The perpendicular magnetic recording system is a recording system that uses a perpendicular disk comprising a laminate of a recording layer having an easy axis and a soft layer perpendicularly, and this system is advantageous in that stronger magnetic filed can be generated in comparison with the conventional system because the soft layer concentrates the magnetic field of a magnetic head. The generated strong recording magnetic field allows us to use a recording layer with high coercivity, and the recording layer with high coercivity allows a small bit to be maintained stable. Furthermore, the perpendicular magnetic recording system is advantageous in that smaller bit reduces demagnetization due to adjacent bits and is stable, and the smaller magnetization pattern can be formed easily the more.
FIG. 20 shows reduction of amplitude with elapsed time of a magnetization pattern recorded by means of the perpendicular magnetization recording system. Herein, the recording density shows exemplary respective recording densities of 10 kFCI, 90 kFCI, and 120 kFCI. In the perpendicular magnetic recording system, the lower recording density reduces the amplitude the more as shown in FIG. 20. A-burst of a servo area is disposed so as to be surrounded by a DC-erasing area, and therefore includes a low recording density corresponding to 10kFCI shown in FIG. 20. Hence, the amplitude of the servo pattern decreases with elapsed time in the case that the conventional servo pattern described with reference to FIG. 18 is applied to the perpendicular magnetic recording system. The servo area cannot be restored by rewriting after shipment because a special sequence is required for recording the servo area.
Furthermore, only the magnetization transition portion generates leak magnetic field in the conventional in-plane magnetic recording system, on the other hand the magnetization pattern itself generates leak magnetic field in the perpendicular magnetic recording system. In other words the DC-signal component generates leak magnetic field extraordinarily, the extraordinary leak magnetic field induces bias field on the entire magnetic head to thereby shift the bias point of a read element and write element. As the result the performance becomes poor. To avoid this disadvantage, a technique that high density pattern is written before a servo pattern is recorded to reduce the effect of DC-signal is proposed in JP-A No. 230734/2002. According to this technique the effect of DC-signal can be reduced, but the effect is effective for only a partial area covering approximately 10 to 30%, namely the residual area obtained by subtracting the width of a write element from a track pitch, in the servo write process.
FIG. 21 shows relation between recording density and overwrite in the perpendicular magnetic recording system. The lower the recording density is, the poorer the overwrite is. It is seen that the low recording density is formed in difficulty. If the conventional servo pattern described with reference to FIG. 18 is applied to the perpendicular magnetic recording system, the magnetic head is deficient in write performance and deficient for saturation recording, and noise component of a disk increases to deteriorate the positioning accuracy. “Servo pattern of first example” and “servo pattern of second example” shown in FIG. 20 will be described hereinafter.
As described hereinabove, combination of the conventional servo pattern and perpendicular magnetic recording system cannot prevent reduction of amplitude with elapsed time in the low recording density situation, generation of extraordinary bias field due to DC-signal component, and deficient write of DC-signal component. As the result, only the magnetic disk apparatus with low reliability is provided.
To solve the problem, it is required that the servo pattern does not include low recording density component. JP-A No. 150729/2002 proposes a servo pattern having a built-in dummy bit with high recording density on the DC-erasing area as shown in FIG. 22. According to this technique the component of frequency included in a servo pattern is increased, and the stability of a recording pattern and overwrite are improved. However, the noise generated from high recording density magnetization is superimposed on the read signal of a servo pattern to cause deterioration of the positioning accuracy. A servo pattern having a dummy bit frequency of double fundamental frequency is recorded, and the track code detection performance is measured. The result is shown in FIG. 23. The deterioration of error rate of track code due to component caused from magnetization of the dummy bit is more serious on the inner radius on which the recording density is higher.
To solve the above-mentioned problem, development of a new technique to realize a magnetic disk apparatus having a tremendous memory capacity of perpendicular magnetic recording system and having an easy-writable and excellent S/N servo pattern of stable long life has been desired.